The expression of surface markers and delineation of functional subpopulations of lymphocytes in the rabbit is being studied. Using immunoelectron-microscopic labeling, the cell surface distribution of lectin and immunoglobulin class markers on different lymphocytes and the effect of activation on the distribution of these markers is being defined. The functional role of lymphocytes in different organs, in particular, those of the gastrointestinal tract are being investigated and better methods of T and B cell fractionation are being developed. Cellular interactions between T and B cells of different rabbits during mixed lymphocyte reactions and the induction of antibody formation are being analyzed. Finally further characterization of a putative population of cells with mixed T and B cell markers is being carried out. These studies should result in a better understanding of the role of different cell types in the immune response of the rabbit as well as lead to a better understanding of the expression of cell surface markers during development and activation of lymphocytes.